In recent years there has been a growing interest in Pb-free solder alloys doped with small amounts of rare earth (RE) elements due to enhanced physical and mechanical properties relative to conventional Pb-free alloys (Ref. 1-17). Several authors have shown that the addition of small amounts of RE elements can decrease alloy melting temperature (Ref. 2, 8, 14), improve wettability (Ref. 2, 5, 9, 16) and even promote strong bonding to semiconductors such as silicon (Ref. 1-13). Rare earths have also been shown to refine solder microstructure by decreasing Sn grain size (Ref. 2, 9, 10, 17), intermetallic particle size (Ref. 2, 9, 15, 17) and decrease the Cu6Sn5 layer that forms between the Cu substrate and the Pb-free solder (Ref. 14, 15). The mechanical response of these materials is dependent on the type of RE elements, concentration, and morphology. RE additions have been shown to increase the strength of Sn—Ag0,0 and Sn—Ag—Cu alloys (Ref. 8, 9, 15, 16), increase strain-to-failure in Sn—Ag0 and Sn—Ag—Cu alloys (Ref. 9), and improve creep resistance in Sn—Ag0 and Sn—Ag Cu alloys (Ref. 7, 16).
In previous work (Ref. 11, 12), we have reported that small La additions (0.1 wt % and 0.5 wt %) to Sn—Ag—Cu resulted in an increase in ductility compared with Sn—Ag—Cu/Cu joints. At these small La concentrations, a homogenously distributed LaSn3 intermetallic phase forms in the solder. We have shown that the LaSn3 particles are directly responsible for the higher ductility observed in these materials, by allowing microscopic voids to nucleate throughout the solder volume (instead of localized strain at the solder-intermetallic interface), and homogenizing the strain in the solder joint.
Due to the reactive nature of RE elements with oxygen, some of these solder systems are prone to severe oxidation even under ambient conditions. In the case of La-containing solders, oxidation leads to degradation in the mechanical performance as well as poor reflow quality (Ref. 18). Chuang and co-authors (Ref. 19-22) have shown that a significant amount of Sn whiskering takes place on the surfaces of Pb-free solders containing Ce, La and Lu. Similar observations have been made by Jiang and Xian (Ref. 23) with solders containing Nd. It appears that the RE-rich intermetallic phases that form in these solders is highly reactive with oxygen leading to a complex oxidation process that produces Sn whiskers. The tendency of RE-containing phases to oxidize in other material systems has also been documented. Niu and co-authors (Ref. 24-26) have shown that RE-containing intermetallics in Fe, Co and Ag systems are susceptible to oxidation. Anzel observed the oxidation of Cu—Er and Cu—Yb phases in Cu alloys (Ref. 27). The La solders, while having increased ductility, the oxidation leads to shear fracturing at the interfaces of the solder joint providing unacceptable performance in electron applications requiring mechanical shock and drop resistance, desirable characteristics that are becoming increasingly important as consumer electronic products become smaller and more portable. The current industry standard lead free solders of Sn-4Ag-0.5Cu and Sn-3Ag-0.5Cu alloys are as much as 40% lower than Pb—Sn alloys in terms of shock performance.